Dodgy front brakes

[JayDee24ca]

The front brake on my rebuilt 79 Convert (still integrated) are pretty weird. I did not rebuild the MC during the build, as it seemed to hold well. However, more times than not, while on the road, when I pull the lever I can pull it 2/3 of the way before it holds. I cannot pump it up by repetitively pumping the lever. When the bike has been sitting, the brake lever is hard and stays that way.
I had long thought that it was just my bleeding of the front brake that was inadequate; I bled it several times and was never truly happy with it for the reasons above. From that description though, does it sound as though the MC simply needs a rebuild?
John D. 

[Tom H]

I'm thinking a warped rotor? While the wheel is turning, the disc is always trying to push the pads open.

 
Tom

[JayDee24ca]

I'm thinking a warped rotor? While the wheel is turning, the disc is always trying to push the pads open.

 
Tom

I should think I would be able to feel pulsation in the lever in that case, right?
JD

[John Croucher]

Warped rotor.  When you apply the brake, one puck moves in, one moves out the same distance as the warped area passes by.  When you release the leaver, the rotor pushes each puck into the caliper.  You then have to push the both pucks out with the lever when braking.

Raise the front wheel, pump up the brake caliper, use some feeler gages to see how far the brakes retract.  Rotate the wheel a couple of turns and then measure again.  You will see that there is a larger gap.  You can put a dial indicator on the rotor and see how much they are out of flatness. 

[Idontwantapickle]

The front brake on my rebuilt 79 Convert (still integrated) are pretty weird. I did not rebuild the MC during the build

It could be a warped rotor but your intuition could be spot on. A clogged port in the MC will cause this symptom as well. Check the rotor and rebuild the master. Also, are the lines original? Many woes await in old brake hoses!

Hunter

[Tom H]

To clarify "the bike sitting".

Your sitting at a stop light and the front brake lever feels firm and normal. Then you start moving again and grab the front brake lever to stop at another light and you have to pull it quite a way in. Then while sitting at the light, you pull the lever again and the brake feels normal again.

I have not had a warped rotor that I know of so I do not have experience with it. You should/may feel it in the lever depending on how bad it is.

EDIT:...One more thing I didn't think about. Floating rotors if your bike has them?? Could the buttons be stuck? COULD?? be they are stuck and not allowing the disc to move and holding the rotor off center pushing the pads open?
EDIT AGAIN:...After looking at some pics, I don't think your rotors are floating design.

I do want to know what solves the problem.

Good luck,
Tom

[Antietam Classic Cycle]

Possibly a bad wheel bearing letting the wheel cock slightly and pushing the pads in?

[pressureangle]

I should think I would be able to feel pulsation in the lever in that case, right?
JD

No, not usually. A warped rotor pushes the pads apart as it turns, but when you apply pressure the pads only see the thickness of the rotor-which does not change- and the whole caliper moves left-right as a unit. This was the standard on many late-80's sportbikes used on the racetrack with pads that didn't like the rotors.

[JayDee24ca]

I'll check the runout on the disc to see. Having owned and driven several 1970 to 1990 Volvos for years I am used to the feel of warped rotors; I went through many sets of them on those cars. I am familiar with what they feel like, so I kinda doubt that this is the problem, but for sure i will check it out, I've been wrong more times than I care to mention. One thing is that it is inconsistent only on the road. But if the bike has been sitting for any time at all, an hour to several days, the lever is always hard. It is only once I am underway that the lever goes intermittently soft, which makes me think master cylinder rebuild. Maybe I am letting my vacuum assist thinking cloud the issue.
I'll post pack for sure.
John D.

[pressureangle]

It's important to verify runout, because the vast majority of 'warped' rotors are not actually warped, but have been overheated and hardened unevenly on the friction surface, creating different coefficients around the rotor which creates a pulsing action, but doesn't actually feed back into the pedal itself.

The only rotor condition I've experienced that actually moves the brake pedal is rotors that are so rusty and/or thin that they spread unevenly. That of course only applies to cars.
Warped motorcycle rotors only move the lever under a light application after spreading the pads, ime.

[JayDee24ca]

Wellp.... Proven wrong once again, and the collective wisdom of the list proven correct again. I put the dial indicator on both sides (for redundancy sake) and found that the front left disc (foot brake) runs true, but the right disc (lever) has 0.030 run out, more than enough to cause the intermittent pull on the lever that I have been experiencing.
Anyone got a good right cast iron disc  to part with? I have a spare, but it is pretty pitted, and I doubt that turning it down to 5.8mm (minimum thickness) will solve it. The one that is on the bike is already pretty thin at 5.78mm.
So, many thanks for the collective input!
John D.

[twowings]

Glad you're 1 step closer to solving the mystery...

[Tom H]

Glad you found it!

Tom

[JayDee24ca]

Reporting back on this, I have achieved only a partial success. I replaced the right front disc with a new one. The runout when checked was still there, but reduced to 0.015, so about half what it was.  I also added lock tabs for the 6 disc bolts, that were missing previously. On the test drive today, the hand lever stays firm, no intermittent half pull, so in that respect I achieved something. However, what is worse, I now have a very aggressive pulse when using that brake, enough that the whole front end shakes badly when braking hard. Not sure what to do next on this. Would a stuck brake pad be to blame for this? This is a worse situation than the original one.
Thanks
John D.

[Chuck in Indiana]

.015 is still not acceptable. Something is wrong. I'd suspect that there is a burr or paint or "something" on the mounting surface causing this. Take the disc off and put your indicator on the mounting surface. It may just need some file work to true it up. Once you get the rotor running true, take some scotch brite and give the swept surface a good scrubbing. Try again.

[JayDee24ca]

Chuck, I fixed it. I put the dial indicator on the hub without the discs and got a variation of 0.007inches on the hub shoulders (which then of course multiplied at the outer circ. of the disc when I put them on). So I got out a good metal file, and proceeded to bring all hub shoulders into line with the lowest one. After mounts and dismounts of the wheel and checking run out,  I got them down to 0.001 on the shoulders, which translated into a 0.003 on the disc at its outer circumference.
I took it for a spin and was amazed how such a neanderthal process could result in such a smooth stop and even pull on the lever.
John D.

[Tom H]

Again, Congrats!!!

Tom

[Huzo]

The blood and guts way to fix it properly is to de lace the wheel and put the hub in a lathe and spin it on it's own axle.
A rolling centre in the headstock and a dead centre in the tailstock, will allow you to spin it on it's own races. A lick off the disc mounting faces will have them running perfectly and the discs will give zero pulsation.
I had to do it with the Bellagio hub when mounting in the Norge for my wheel conversion.
You may not want to be bothered, but if you search "Be your own spokesperson", the process is covered in post #9.

[JayDee24ca]

The blood and guts way to fix it properly is to de lace the wheel and put the hub in a lathe and spin it on it's own axle.
A rolling centre in the headstock and a dead centre in the tailstock, will allow you to spin it on it's own races. A lick off the disc mounting faces will have them running perfectly and the discs will give zero pulsation.
I had to do it with the Bellagio hub when mounting in the Norge for my wheel conversion.
You may not want to be bothered, but if you search "Be your own spokesperson", the process is covered in post #9.

Huzo, perhaps I should have mentioned that this is on a mag wheel and not on a spoked hub. Good advice nonetheless, thanks.
JD

[Chuck in Indiana]

Chuck, I fixed it. I put the dial indicator on the hub without the discs and got a variation of 0.007inches on the hub shoulders (which then of course multiplied at the outer circ. of the disc when I put them on). So I got out a good metal file, and proceeded to bring all hub shoulders into line with the lowest one. After mounts and dismounts of the wheel and checking run out,  I got them down to 0.001 on the shoulders, which translated into a 0.003 on the disc at its outer circumference.
I took it for a spin and was amazed how such a neanderthal process could result in such a smooth stop and even pull on the lever.
John D.

Attaboy! Glad it worked for you. Sometimes a guy has to do what a guy has to do with what he has.

[Huzo]

Huzo, perhaps I should have mentioned that this is on a mag wheel and not on a spoked hub. Good advice nonetheless, thanks.
JD

No worries..
I have seen a 17" wheel mounted and clocked true in a lathe for another operation.
Doing this with a dial indicator on the rim and a thou' or two off the disc mount faces and you're home and hosed..